1. Field of the Invention
The present invention relates to a buffer device for an elevator that uses a hydraulic buffer for alleviating shock generated when a traveling (ascending/descending) body impacts the bottom of a hoistway.
2. Description of the Related Art
FIG. 18 is a construction diagram showing an example of a conventional elevator. In the upper portion of a hoistway 1, there is a hoisting machine 3 having a driving sheave 2 and a deflector sheave 4, and a main rope (hoisting rope) 5 is wrapped around the driving sheave 2 and the deflector sheave 4. From one end portion of the main rope 5, a car 6 as a traveling body is suspended. From the other end portion of the main rope 5, a counterweight 7 that is another traveling body is suspended. Normally, the weight of the counterweight 7 is set so as to be equal to the sum of the own weight of the car 6 per se and 50% of the rated load capacity of the car 6.
At the bottom (pit) of the hoistway 1, a car buffer 8 and a counterweight buffer 9 are installed. The car buffer 8 and the counterweight buffer 9 alleviate shock generated when the car 6 or the counterweight 7 collide with the bottom of the hoistway 1. Although the car buffer 8 and the counterweight buffer 9 can be broadly classified into spring buffers and hydraulic buffers, if the rated speed of an elevator is equal to 90 m/minor more, a hydraulic buffer is used for the elevator.
FIG. 19 is a front view showing an example of a conventional hydraulic buffer. On an attachment base 11, a cylinder 12 filled with oil is provided. Into this cylinder 12, there is inserted a cylindrical plunger 13 that is capable of reciprocating in an axial direction. On the upper end portion of the cylinder 12, a flange 14 is fixed. On the upper end portion of the plunger 13, a spring bracket 15 is fixed.
Between the flange 14 and the spring bracket 15, there is arranged a return spring 16 that urges the plunger 13 in a direction (upward direction) in which the plunger 13 protrudes from the cylinder 12. In order to avoid a metal-to-metal impact that occurs when the car 6 or the counterweight 7 impacts the hydraulic buffer, a buffer member 17 is provided on the spring bracket 15.
FIG. 20 is a cross-sectional view that schematically shows the internal construction of the hydraulic buffer in FIG. 19. In the lower portion of the plunger 13, an orifice 18 is provided. In the cylinder 12, a control rod 19 is fixed. The control rod 19 is inserted into the plunger 13 from the orifice 18 when the plunger 13 is moved downward.
Also, the diameter of the control rod 19 is changed in the axial direction (vertical direction). Consequently, the clearance area between the orifice 18 and the control rod 19 changes in accordance with the amount of displacement of the plunger 13. That is, the diameter of the control rod 19 gradually increases in a downward direction and, when the amount of downward displacement of the plunger 13 increases, the clearance between the orifice 18 and the control rod 19 is narrowed. As a result, a reaction force generated by hydraulic pressure acts on the plunger 13 and the impacting car 6 or counterweight 7 is decelerated.
The hydraulic buffer is designed so that when the car 6 collides at a speed that is 1.15 times faster than the rated speed, the car 6 is decelerated at a predetermined rate and is stopped with safety. As a result, in accordance with increases in the rated speed, the stroke of the plunger 13 is elongated and therefore the height of the hydraulic buffer is increased.
If the height of the hydraulic buffer is increased as described above, the depth of a pit in which the hydraulic buffer is contained is also increased. In view of this problem, for the sake of reducing pit depth, it is permitted by US rules (ASME 17.1a-1997 Rule 201.4h) that a part of the plunger 13 can be positioned in the traveling path of the car 6 during normal operation. That is, under this US rule, when the car 6 lands at the lowest floor, the car 6 is allowed to displace within a range of ¼ or less of the whole stroke of the plunger 13.
In this case, each time the car 6 lands at the lowest floor during normal operation, the car 6 impacts the hydraulic buffer. However, the speed, at which the car 6 impacts the hydraulic buffer during normal operation, becomes considerably lower than a speed at the time when the hydraulic buffer functions as a safety apparatus, so that the level of shock is also reduced.
FIG. 21 is a cross-sectional view showing a main portion of another example of a conventional hydraulic buffer. In this example, on the upper end portion of the plunger 13, there are mounted a buffer member 21 and an auxiliary buffer 22. The auxiliary buffer 22 includes a cylinder 23, a piston rod 24 inserted into the cylinder 23, a piston 25 that is fixed on the tip portion of the piston rod 24 and is made to slide within the cylinder 23, a supporting plate 26 that is fixed on the base end portion of the piston rod 24 and is coupled to the upper end portion of the buffer member 21, and a free piston 27 that is arranged within the cylinder 23.
Between the piston 25 and the free piston 27 within the cylinder 23, there is formed a lower portion oil chamber 28. Above the piston 25 within the cylinder 23, there is formed an upper portion oil chamber 29. Below the free piston 27 within the cylinder 23, there is formed a gas chamber 30. The piston 25 is provided with a check valve 31 and an orifice 32 (see JP 2001-241506 A, for instance).
In a hydraulic buffer like this, when there is an impact of a car 6, the buffer member 21 is compressed and the piston rod 24 is displaced downward. Following this, the buffer member 21 tries to restore its initial state in a decompression direction, although rapid restoration of the buffer member 21 is prevented by the auxiliary buffer 22. As a result, vibration of the buffer member 21 is prevented and therefore a situation where a passenger in the car 6 feels discomfort due to the vibration can be avoided.
In the conventional hydraulic buffer constructed in the manner described above, as a material of the buffer member 17, there is selected a material that possess high stiffness which is able to stand the weight of the car 6 and the reaction force of hydraulic pressure from the plunger 13. Therefore, when the car 6 impacts the hydraulic buffer, shock and noise are generated. In particular, in elevators where the car 6 impacts the hydraulic buffer even during normal operation, there is a danger that a passenger will feel discomfort due to the shock and noise generated by the impact.
It is possible to alleviate such shock and noise to some extent by making the buffer member 17 thick and soft, although if the thickness of the buffer member 17 is increased, the height of the buffer under a compressed state is also increased accordingly, which leads to a situation where the depth (pit depth) from the bottom surface of the car 6 to the bottom of the hoistway 1 when the car 6 is positioned at the lowest floor is increased.
Also, in cases where the auxiliary buffer 22 shown in FIG. 21 is provided, the pit depth is increased because the auxiliary buffer 22 is thick. Further, the auxiliary buffer 22 is provided to suppress the vibration of the buffer member 21, so that the shock at the time of impact with the buffer member 21 is not sufficiently alleviated.